Sprint has not yet officially named Boston as one of the cities offering the 4g service but I can confirm that it is available.
I was able to pick up a 4g signal on my HTC EVO in several parts of Boston.
The only place that I was able to maintain a connection was in one of my client’s buildings in the south end. I did see a blip of 4g at Wellington station and while driving around Boston but could not stay connected.
I ran a speed test and got speeds of 3.5 down and 2.5 up!
You can check the Sprint’s 4g coverage map here
PS the Iphone 4 is not running on a 4g network, at least not in the states anyway. Sprint is currently the only carrier providing 4g coverage and the HTC EVO is the only 4g phone available.
You can still get an Iphone 4 if you don’t mind duct taping it to receive calls.
If you’re still thinking about getting an Iphone 4 see this video below.